Synopsis: Hidden Structure of Plasmons

Calculations of the current density within collective charge oscillations called plasmons reveal a complicated structure that could affect how plasmons reflect off a boundary.

L.-k. Shi et al., Phys. Rev. X (2018)

Plasmons are collective oscillations of the electric charges within a material. Models of plasmons typically describe them as simple waves in the electron density, but new theoretical work suggests that the currents within plasmons could be more complex than expected. Li-kun Shi and Justin Song from the Agency for Science, Technology and Research in Singapore investigated plasmons in a 2D metal exposed to a magnetic field. Their calculations show that the local current density can develop textures that resemble the hedgehog-like spin configurations in certain magnetic materials. This hidden structure may lead to a shift in the position of a plasmon when it reflects off a boundary.

Recent interest in plasmons stems from their short wavelengths and strong coupling to light, which could prove useful in imaging and on-chip data transfer. In their study, Shi and Song focused on the currents that swirl around in a plasmon, representing the localized current at each point as a “spin” pointing in the direction of current flow. Their calculations indicate that these spins rotate in a magnetic field, creating a hedgehog-like spatial pattern that can be described by a geometric phase. Such a phase can cause the plasmons to shift by a small transverse distance when they reflect off a boundary. A similar skipping occurs in the Hall effect of light, in which a geometric phase in the photon polarization leads to a transverse shift in reflection. Shi and Song believe that the plasmon shift could be observed in graphene and other 2D materials, where it might be used to realize a filter that blocks certain plasmons by shifting them toward an absorbing material.